1. Field of the Invention
The present invention relates to a laser-diode-pumped solid-state laser apparatus. The present invention also relates to a laser-diode-pumped fiber laser apparatus. The present invention further relates to a laser-diode-pumped fiber laser amplifier.
2. Description of the Related Art
(1) Ultraviolet Laser
Highly efficient, high output power ultraviolet lasers which continuously oscillate in the ultraviolet wavelength range are required, for example, for applications in ultraviolet lithography, fluorometric analysis of organic cells, and the like. GaN-based compound semiconductor lasers having an active layer made of an InGaN, InGaNAs, or GaNAs material are known as lasers which oscillate in the ultraviolet wavelength range. Recently, GaN-based compound semiconductor lasers which can continuously oscillate for a thousand hours at the wavelength of 400 nm with output power of several milliwatts are provided.
However, the current GaN-based compound semiconductor lasers cannot emit laser light with output power of 100 mW or more by a single transverse mode, which is required in many applications. In addition, the oscillation efficiency in the current GaN-based compound semiconductor lasers which emit laser light having wavelengths of 380 nm or below is low, and the lifetimes of such semiconductor lasers are very short.
On the other hand, wavelength-conversion solid-state lasers which output ultraviolet laser beams having wavelengths of 400 nm or below are known. In these wavelength-conversion solid-state lasers, wavelengths of laser oscillation light are shortened to the ultraviolet wavelengths by second harmonic generation (SHG) or third harmonic generation (THG) using nonlinear optical crystals.
However, solid-state laser mediums which can efficiently oscillate in the wavelength range of 700 to 800 nm have not yet been found. Therefore, it is difficult to obtain high output power from the wavelength-conversion solid-state lasers in which the wavelengths of the laser light are shortened by second harmonic generation (SHG).
In addition, the efficiency of the wavelength-conversion solid-state lasers in which the wavelengths of the laser light are shortened by third harmonic generation (THG) is essentially low, and the current THG wavelength-conversion solid-state lasers can oscillate in only a pulse mode. In order to realize continuous oscillation, it is necessary to maintain resonance of SHG light of the fundamental wave, and highly accurate temperature adjustment of a resonator with a precision of 0.01iC is required for oscillation of the THG light. However, such accurate temperature adjustment is practically difficult in terms of cost.
(2) Blue and Green Lasers
Gas-laser-pumped solid-state laser apparatuses in which a Pr3+-doped solid-state laser crystal is pumped with a gas laser such as an Ar laser are known as disclosed in Journal of Applied Physics, vol. 48, No. 2, pp.650-653 (1977), and Applied Physics, B58, pp.149-151 (1994). In addition, a solid-state laser apparatus in which a Pr3+-doped solid-state laser crystal is pumped by second harmonic (SH) light from a lamp-pumped solid-state laser is known, as disclosed in xe2x80x9cAdvanced Solid-State Laser,xe2x80x9d OSA TOPS, vol.19, pp.34-35, the Optical Society of America, 1998. In these solid-state laser apparatuses, it is possible to generate a laser beam in a blue wavelength range of 470 to 490 nm by a transition from 3P0 to 3H4. It is also possible to generate a laser beam in a green wavelength range of 520 to 550 nm by a transition from 3P1 to 3H1. Therefore, the above solid-state laser apparatuses can be used as a light source for recording a color image in a color sensitive material.
The light sources for use in recording a color image in a color sensitive material are required to be small in size, light in weight, and inexpensive. However, the above gas-laser-pumped and lamp-pumped solid-state laser apparatuses using the Pr3+-doped solid-state laser crystals are not suitable for use in recording a color image in a color sensitive material since the pumping light sources in these solid-state laser apparatuses are large, heavy, and expensive.
In addition, a solid-state laser apparatus in which a Pr3+-doped solid-state laser crystal is pumped by a blue laser beam emitted from an SHG laser apparatus is known as disclosed by Andy Clarkson, xe2x80x9cVisible and UV Sources,xe2x80x9d Technical Digest of CLEO ""99, University of Southampton, 1999.
However, the above SHG-pumped solid-state laser apparatuses using the Pr3+-doped solid-state laser crystals are also not suitable for use in recording a color image in a color sensitive material since the pumping light sources in the SHG-pumped solid-state lasers are large, heavy, and expensive.
As another solid-state laser apparatus which emits a laser beam having a wavelength in the blue or green wavelength range, Japanese Unexamined Patent Publication No. 4(1992)-318988 discloses a laser-diode-pumped SHG laser apparatus in which a solid-state laser beam is converted into a second harmonic, i.e., the wavelength of the solid-state laser beam is reduced by half by arranging a nonlinear optical crystal in a resonator.
However, the efficiency of wavelength conversion in the current laser-diode-pumped SHG laser apparatuses in which a wavelength of a solid-state laser beam is reduced by using a nonlinear optical crystal is not sufficiently high, and therefore it is difficult to obtain high output power. In addition, in such laser-diode-pumped SHG laser apparatuses, an etalon or the like is inserted for limiting the oscillation mode to a single mode. Therefore, loss in the resonator is great, and thus achievement of high output power becomes more difficult. Further, in order to match phases in the wavelength conversion, highly accurate temperature control is required, and therefore the outputs of the laser-diode-pumped SHG laser apparatuses are not stable. Moreover, since the numbers of parts are increased by the provision of the nonlinear optical crystal and the etalon, the laser-diode-pumped SHG laser apparatuses are expensive.
Recently, InGaN-based compound laser diodes and ZnMgSSe-based compound laser diodes which emit laser beams in the blue and green wavelength ranges have been developed.
Since, the oscillation wavelengths of the InGaN-based compound laser diodes increase with increase in the indium content, theoretically it is possible to obtain laser beams in the blue wavelength range of 470 to 490 nm, or laser beams in the green wavelength range of 520 to 550 nm. However, since the quality of the crystal deteriorates with the increase in the indium content, it is practically impossible to sufficiently increase the indium content, and the upper limit of the lengthened wavelength is about 450 nm.
In addition, blue light can be obtained by laser diodes having an active layer made of an InGaNAs or GaNAs material. The oscillation wavelengths in these laser diodes can also be increased by doping the active layer with arsenic. However, since the quality of the crystal deteriorates with the increase in the arsenic content, the upper limit of the wavelength realizing high output power is about 450 to 460 nm.
Further, the current ZnMgSSe-based compound laser diodes cannot oscillate continuously at wavelengths below 500 nm at room temperature, and the lifetimes are at most a hundred hours.
Japanese Unexamined Patent Publication No. 11(1999)-17266, which is assigned to the present assignee, discloses a laser-diode-pumped solid-state laser apparatus which is inexpensive, and can emit a laser beam in the blue or green wavelength range with high efficiency, high output power, and high output stability. In this laser-diode-pumped solid-state laser apparatus, a Pr3+-doped solid-state laser crystal is pumped with an InGaN-based compound laser diode having an active layer made of an InGaN-based compound, or an InGaNAs-based compound laser diode having an active layer made of an InGaNAs-based compound, or a GaNAs-based compound laser diode having an active layer made of a GaNAs-based compound. However, in the above laser-diode-pumped solid-state laser apparatus, the amount of the pumping light absorbed by the Pr3+-doped solid-state laser crystal is not sufficient.
(3) Fiber Lasers
As disclosed in the Technical Report of the Institute of Electronics, information and Communication Engineers in Japan, LQE95-30 (1995) p.30, and Optics Communications 86 (1991) p.337, laser-diode-pumped fiber laser apparatuses in which an optical fiber having a core made of a Pr3+-doped fluoride is pumped with a laser diode so as to generate a laser beam are known.
In addition, the above references also disclose an optical fiber amplifier in which an optical fiber having a Pr3+-doped core is pumped with a laser diode so as to generate fluorescent light, and incident light of the optical fiber is amplified by the energy of the fluorescent light when the wavelength of the incident light is included in the wavelength range of the fluorescent light.
Besides, an Ar-laser-pumped, Pr3+-doped fiber laser apparatus is also disclosed in Optics Communications 86 (1991) p.337, which reports laser oscillations at the wavelengths of 491, 520, 605, and 635 nm with pumping light having a wavelength of 476.5 nm.
Further, Japanese Patent Application No. 9(1997)-110554, which is assigned to the present assignee, discloses a laser-diode-pumped solid-state laser apparatus in which a Pr3+-doped solid-state laser crystal is pumped with a laser diode.
The above laser-diode-pumped fiber laser apparatuses, Ar-laser-pumped, Pr3+-doped fiber laser apparatus, and laser-diode-pumped solid-state laser apparatus can emit blue or green laser beams, and the above optical fiber amplifier can amplify blue or green laser beams. In this respect, it is considered that these apparatuses may be used as constituents of light sources for recording a color image in a color sensitive material.
However, a water cooling system is required for operating the Ar-laser-pumped, Pr3+-doped fiber laser apparatus with high power of a few watts to several tens of watts. Therefore, the size is increased, and the lifetime and the efficiency are reduced.
On the other hand, since the laser-diode-pumped solid-state laser apparatuses in which a Pr3+-doped solid-state laser crystal is pumped with a laser diode has such structure that thermal energy of pumping light is concentrated in a small solid-state laser crystal, absorption of the thermal energy by the crystal causes heat generation and a thermal lens effect. Therefore, beam quality and output stability deteriorate. In particular, the deterioration is serious in the operation with high power of a few watts to several tens of watts.
Japanese Unexamined Patent Publication No. 11(1999)-204862, which is assigned to the present assignee, discloses a fiber laser apparatus which can be realized in a small size, and can emit a blue or green laser beam with high efficiency, high output power, and highly stable quality and output level. In this fiber laser apparatus, an optical fiber having a Pr3+-doped core is pumped with a GaNAs-based compound laser diode.
In addition, JPP No. 11-204862 also discloses a fiber laser amplifier which can be realized in a small size, and can amplify a blue or green laser beam with high efficiency and highly stable quality and output level. In this fiber laser amplifier, a first optical fiber having a Pr3+-doped core is pumped with a GaNAs-based compound laser diode so as to generate fluorescent light, and incident light of the optical fiber is amplified by the energy of the fluorescent light when the wavelength of the incident light is included in the wavelength range of the fluorescent light.
However, in the above fiber laser apparatus and fiber laser amplifier disclosed in JPP No. 11-204862, the amount of the pumping light absorbed by the fiber core is insufficient.
An object of the present invention is to provide a semiconductor-laser-pumped solid-state laser apparatus, which can continuously emit ultraviolet laser light having the wavelengths of about 360 nm or below with high efficiency and high output power, and can be produced at low cost.
Another object of the present invention is to provide a laser-diode-pumped solid-state laser apparatus, in which a solid-state laser crystal is pumped with a laser diode having an active layer made of an InGaN, InGaNAs, or GaNAs material, an amount of pumping light absorbed by the solid-state laser crystal is increased, and high efficiency and high output power are achieved.
Still another object of the present invention is to provide a fiber laser apparatus, in which a solid-state laser crystal is pumped with a GaN-based compound laser diode, an amount of pumping light absorbed by a fiber core is increased, and high efficiency and high output power are achieved.
A further object of the present invention is to provide a fiber laser amplifier, in which a solid-state laser crystal is pumped with a GaN-based compound laser diode, an amount of pumping light absorbed by a fiber core is increased, and a high amplifier gain is achieved.
(1) According to the first aspect of the present invention, there is provided a laser-diode-pumped solid-state laser apparatus including a laser diode which has an active layer made of one of an InGaN, InGaNAs, and GaNAs materials, and emits a pumping laser beam; a solid-state laser crystal which is doped with at least one rare-earth element including at least Pr3+, and emits solid-state laser light when the solid-state laser crystal is pumped by the pumping laser beam; and an optical wavelength conversion element which converts the solid-state laser light into ultraviolet laser light by wavelength conversion.
Preferably, the laser-diode-pumped solid-state laser apparatus according to the first aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iii).
(i) The solid-state laser crystal may be codoped with Pr3+ and at least one of Er3+, Ho3+, Dy3+, Eu3+, Sm3+, Pm3+, and Nd3+.
(ii) The solid-state laser crystal may emit a solid-state laser beam having a wavelength of about 720 nm by a transition from 3P0 to 3F4 in the solid-state laser crystal, and the ultraviolet light may be a second harmonic of the solid-state laser light, and have a wavelength of about 360 nm.
(iii) The optical wavelength conversion element may be made of a nonlinear optical crystal having periodic domain-inverted structure.
When a Pr3+-doped solid-state laser crystal such as a Pr3+:YLF crystal is pumped with a GaN-based compound laser diode, the Pr3+-doped solid-state laser crystal efficiently oscillates in the wavelength range of 700 to 800 nm. For example, the Pr3+-doped solid-state laser crystal efficiently emits a solid-state laser beam having the wavelength of 720 nm by a transition from 3 to 3F4, where the wavelength of 720 nm corresponds to an oscillation peak of Pr3+ in the infrared range. In this case, high intensity ultraviolet light having the wavelength of 360 nm is obtained by wavelength conversion of the solid-state laser beam having the wavelength of 720 nm into a second harmonic by using an optical wavelength conversion element.
In addition, it is relatively easy for the GaN-based compound laser diodes to emit laser beams in the wavelength range of 380 to 430 nm, and each of the dopants, Er3+, Ho3+, Dy3+, Eu3+, Sm3+, Pm3+, and Nd3+, has an absorption band in the wavelength range of 380 to 430 nm. Therefore, these dopants can be pumped with the GaN-based compound laser diodes. In the solid-state laser crystal codoped with Pr3+ and at least one of Er3+, Ho3+, Dy3+, Eu3+, Sm3+, Pm3+, and Nd3+, the excited electrons can move to an excitation level (e.g., 3P0 to 3P1) of Pr3+. Then, the solid-state laser crystal can emit a solid-state laser beam corresponding to an oscillation peak of Pr3+, i.e., corresponding to a transition between two energy levels of Pr3+. For example, the solid-state laser crystal can emit a solid-state laser beam having the wavelength of 720 nm by a transition from 3P0 to 3F4, or the like. In this case, high intensity ultraviolet light having the wavelength of 360 nm can be obtained by wavelength conversion of the solid-state laser beam having the wavelength of 720 nm into a second harmonic by using an optical wavelength conversion element.
As mentioned above, it is relatively easy for the GaN-based compound laser diodes to emit laser beams in the wavelength range of 380 to 430 nm. In particular, in the wavelength range of 400 to 410 nm, the output power of the currently available GaN-based compound laser diode is maximized. Therefore, since Er3+, Ho3+, Dy3+, Eu3+, Sm3+, Pm3+, and Nd3+ are pumped with the GaN-based compound laser diodes, the absorbed amount of the pumping light is increased, and thus high efficiency and high output power can be achieved.
In particular, when the optical wavelength conversion element performs wavelength conversion into a second harmonic, the construction for performing the wavelength conversion can be simplified, compared with the construction of wavelength conversion into a third harmonic, and therefore a laser-diode-pumped solid-state laser apparatus can be realized at low cost.
In addition, the thermoconductivity of the GaN-based compound laser diodes is very great (i.e., about 130 W/mxc2x0 C.), compared with the thermoconductivity of the other laser diodes such as ZnMgSSe-based compound laser diodes (which have thermoconductivity of 4 W/mxc2x0 C.). Further, since the dislocation mobility of the GaN-based compound laser diodes is very low, compared with that of ZnMgSSe-based compound laser diodes, the COD (catastrophic optical damage) thresholds of the GaN-based compound laser diodes are very high. Therefore, it is easy to achieve a long lifetime and high output power. Since the laser-diode-pumped solid-state laser apparatus according to the first aspect of the present invention uses a GaN-based compound laser diode as a pumping light source, the laser-diode-pumped solid-state laser apparatus can also have a long lifetime, and emit an ultraviolet laser beam with high output power.
(2) According to the second aspect of the present invention, there is provided a laser-diode-pumped solid-state laser apparatus including a laser diode which has an active layer made of one of an InGaN, InGaNAs, and GaNAs materials, and emits a pumping laser beam; and a solid-state laser crystal which is codoped with Pr3+ and at least one of Er3+, Ho3+, Dy3+, Eu3+, Sm3+, Pm3+, and Nd3+, and emits solid-state laser light when the solid-state laser crystal is pumped by the pumping laser beam.
Preferably, the laser-diode-pumped solid-state laser apparatus according to the second aspect of the present invention may also have one or any possible combination of the following additional features (iv) to (vi)(iv)
The solid-state laser light may have a blue wavelength in a range of 465 to 495 nm.
(v) The solid-state laser light may have a green wavelength in a range of 515 to 555 nm.
(vi) The solid-state laser light may have a red wavelength in a range of 600 to 660 nm.
Since each of the dopants, Er3+, Ho3+, Dy3+, Eu3+, Sm3+, Pm3+, and Nd3+, has an absorption band in the wavelength range of 380 to 430 nm, these dopants can be pumped with the GaN-based compound laser diodes. Therefore, in the solid-state laser crystal codoped with Pr3+ and at least one of Er3+, Ho3+, Dy3+, Eu3+, Sm3+, Pm3+, and Nd3+, the excited electrons can move to excitation levels (e.g., 3P0 to 3P1) of Pr3+, and the solid-state laser crystal can emit blue, green, and red laser beams corresponding to oscillation peaks of Pr3+ by transitions between the energy levels of Pr3+.
As mentioned before, it is relatively easy for the GaN-based compound laser diodes to emit laser beams in the wavelength range of 380 to 430 nm. In particular, in the wavelength range of 400 to 410 nm, the output power of the currently available GaN-based compound laser diodes is maximized. Therefore, since Er3+, Ho3+, Dy3+, Eu3+, Sm3+, Pm3+, and Nd3+ are pumped with the GaN-based compound laser diodes, the absorbed amount of the pumping light is increased, and thus high efficiency and high output power can be achieved.
In addition, for the same reasons as explained for the first aspect of the present invention, the laser-diode-pumped solid-state laser apparatus according to the second aspect of the present invention can have a long lifetime, and emit a laser beam in the blue and green wavelength ranges with high output power.
The GaN-based compound laser diodes used as a pumping light source may be a single longitudinal or transverse mode, broad-area, phased-array, or MOPA (master oscillator power amplifier) type high power laser diode. In addition, one or more GaN-based compound laser diodes may be used in a laser-diode-pumped solid-state laser apparatus. Thus, the laser-diode-pumped solid-state laser apparatus according to the second aspect of the present invention can emit a laser beam with an even higher output power, e.g., in the order of 1 W.
In addition, since the laser-diode-pumped solid-state laser apparatus according to the second aspect of the present invention does not require a nonlinear optical crystal, an etalon or the like, the number of constituents of the laser-diode-pumped solid-state laser apparatus is small, i.e., the construction of the laser-diode-pumped solid-state laser apparatus can be simplified, and the operation is stable in a wide temperature range. Therefore, the laser-diode-pumped solid-state laser apparatus according to the second aspect of the present invention can be used as a stable and inexpensive pumping light source.
Further, since the laser-diode-pumped solid-state laser apparatus according to the second aspect of the present invention does not perform wavelength conversion, highly accurate temperature control for phase matching is unnecessary. Therefore, it is possible to avoid the output instability due to the temperature control, i.e., high output stability can be achieved. (3) According to the third aspect of the present invention, there is provided a fiber laser apparatus including a GaN-based compound laser diode which emits a pumping laser beam; and an optical fiber which is codoped with Pr3+ and at least one of Er3+, Ho3+, Dy3+, Eu3+, Sm3+, Pm3+, and Nd3+, and emits laser light when the optical fiber is pumped by the pumping laser beam.
Preferably, the fiber laser apparatus according to the third aspect of the present invention may also have one or any possible combination of the following additional features (vii) to (x).
(vii) The laser light may have a blue wavelength in a range of 465 to 495 nm.
(viii) The laser light may have a green wavelength in a range of 515 to 555 nm.
(ix) The laser light may have a red wavelength in a range of 600 to 660 nm.
(x) The GaN-based compound laser diode may have an active layer made of one of an InGaN, InGaNAs, and GaNAs materials.
The fiber laser apparatus according to the third aspect of the present invention has basically the same advantages as the laser-diode-pumped solid-state laser apparatus according to the second aspect of the present invention.
(4) According to the fourth aspect of the present invention, there is provided a fiber laser amplifier including a GaN-based compound laser diode which emits a pumping laser beam; and an optical fiber which is codoped with Pr3+ and at least one of Er3+, Ho3+, Dy3+, Eu3+, Sm3+, Pm3+, and Nd3+, generates fluorescence with at least one wavelength when the optical fiber is pumped by the pumping laser beam, and amplifies incident light which has a wavelength included in the at least one wavelength of the fluorescence.
Preferably, the fiber laser amplifier according to the fourth aspect of the present invention may also have one or any possible combination of the following additional features (xi) to (xiv).
(xi) The fluorescence may have a blue wavelength in a range of 465 to 495 nm.
(xii) The fluorescence may have a green wavelength in a range of 515 to 555 nm.
(xiii) The fluorescence may have a red wavelength in a range of 600 to 660 nm.
(xiv) The GaN-based compound laser diode may have an active layer made of one of an InGaN, InGaNAs, and GaNAs materials.
The fiber laser amplifier according to the fourth aspect of the present invention has basically the same advantages as laser-diode-pumped solid-state laser apparatus according to the second aspect of the present invention.